Phase Transition to an Opaque Plasma in a Sonoluminescing Bubble

Time-resolved spectrum measurements of a sonoluminescing Xe bubble reveal a transition from transparency to an opaque Planck blackbody. As the temperature is $<10000\mathrm{K}$ and the density is below liquid density, the photon scattering length is 10 000 times too large to explain its opacity. We resolve this issue with a model that reduces the ionization potential. According to this model, sonoluminescence originates in a new phase of matter with high ionization. Analysis of line emission from ${\mathrm{Xe}}^{*}$ also yields evidence of phase segregation for this first-order transition inside a bubble.

Figure 1

Solutions to Eqs. (1, 2) for the degree of ionization $x$ as a function density of Xe atoms for various temperatures. The lower branch solid lines correspond to a minimum of the free energy, and the upper branch dashed lines correspond to a maximum as indicated by the inset which displays a plot of the free energy as a function of $x$ for point 6. Note that the density $\sim {10}^{22}/{\mathrm{cm}}^3$ of single bubble sonoluminescence at 40 kHz lies to the right of these curves.

Figure 2

Time-resolved spectra for a sonoluminescing xenon bubble pulsating in phosphoric acid at 40 Hz (see the section on methods). The time between successive spectra is 100 ns and spectrum no. 5 coincides with the minimum radius and occurs 500 ns into the flash. The spectra have contributions from a thermal continuum as well as a line at 823 nm. The quality of the blackbody fit to the thermal portion of the spectrum is equally good for the data near $R_c$ as well as the data where the 823 line is prominent. Each acquisition is 10 ns in duration, and the graph is the average of 100 acquisitions in each of 4 different grating positions. The inset shows the best blackbody fits to the continuum and the top of the 823 nm spectral line at point no. 9 which occurs 400 ns after the minimum radius when the temperature is 6200 K. The blackbody fits differ only in the radius of the region of emission, $R_{\mathrm{bb}}=40\mu \mathrm{m}$ and $R^{*}=80\mu \mathrm{m}$.

Figure 3

Radii and blackbody temperature for a sonoluminescing bubble at 40 Hz. The radius of the wall of the bubble $R$ is obtained with a backlit streak image and is shown in black. The blackbody temperature and radius $R_{\mathrm{bb}}$ obtained from fitting the thermal portion of the spectra in Fig. 2 is shown in red and blue, respectively. The radius $R^{*}$ of the blackbody whose spectrum passes through the peak of the emission at 823 nm at the temperature determined by the red line is shown in green. Horizontal error bars represent the uncertainty in synchronizing spectrums and streak images in time.